ABSTRACTBacillus thuringiensisCry1Ab and Cry1Fa toxins are environmentally safe insecticides that control important insect pests.Spodoptera frugiperdais an important maize pest that shows low susceptibility to Cry1A toxins, in contrast to Cry1Fa, which is highly active against this pest and is used in transgenic maize forS. frugiperdacontrol. The β16 region from domain III of Cry1Ab has been shown to be involved in interactions with receptors such as alkaline phosphatase (ALP) or aminopeptidase (APN) in different lepidopteran insects. Alanine-scanning mutagenesis of amino acids of Cry1Ab β16 (509STLRVN514) revealed that certain β16 mutations, such as N514A, resulted in increased toxicity of Cry1Ab forS. frugiperdawithout affecting the toxicity for other lepidopteran larvae, such asManduca sextalarvae. Exhaustive mutagenesis of N514 was performed, showing that the Cry1Ab N514F, N514H, N514K, N514L, N514Q, and N514S mutations increased the toxicity towardS. frugiperda. A corresponding mutation was constructed in Cry1Fa (N507A). Toxicity assays of wild-type and mutant toxins (Cry1Ab, Cry1AbN514A, Cry1AbN514F, Cry1Fa, and Cry1FaN507A) against fourS. frugiperdapopulations from Mexico and one from Brazil revealed that Cry1AbN514A and Cry1FaN507A consistently showed 3- to 18-fold increased toxicity against four of fiveS. frugiperdapopulations. In contrast, Cry1AbN514F showed increased toxicity in only two of theS. frugiperdapopulations analyzed. The mutants Cry1AbN514A and Cry1AbN514F showed greater stability to midgut protease treatment. In addition, binding analysis of the Cry1Ab mutants showed that the increased toxicity correlated with increased binding to brush border membrane vesicles and increased binding affinity forS. frugiperdaALP, APN, and cadherin receptors.IMPORTANCESpodoptera frugiperdais the main maize pest in South and North America and also is an invasive pest in different African countries. However, it is poorly controlled byBacillus thuringiensisCry1A toxins expressed in transgenic crops, which effectively control other lepidopteran pests. In contrast, maize expressing Cry1Fa is effective in the control ofS. frugiperda, although its effectiveness is being lost due to resistance evolution. Some of the Cry1Ab domain III mutants characterized here show enhanced toxicity forS. frugiperdawithout loss of toxicity toManduca sexta. Thus, these Cry1Ab mutants could provide useful engineered toxins that, along with other Cry toxins, would be useful for developing transgenic maize expressing stacked proteins for the effective control ofS. frugiperdaand other lepidopteran pests in the field.
Ovipositional responses of Spodoptera frugiperda on host plants provide a basis for using Bt-transgenic maize as trap crop in China
